The present invention relates to diodes, and more particularly to diodes having a structure, in which a semiconductor crystal has been epitaxially grown in narrow grooves.
FIG. 21 is a plan view of a conventional diode 101, and FIG. 22 is a cross-sectional view taken along the line Pxe2x80x94P in FIG. 21.
This diode 101 includes an N-type silicon substrate 111. An N-type epitaxial layer 112 is formed on the surface of the silicon substrate 111.
The grooves whose planar shape is that of a rectangular ring surface are provided on the surface of the epitaxial layer 112. In the case shown, three rectangular ring-shaped grooves are provided, and these ring-shaped grooves are arranged concentrically. The ring-shaped grooves are each filled with a semiconductor layer containing P-type impurities and formed by epitaxial growth.
The innermost of these rectangular ring-shaped semiconductor layers is an intermediate withstand voltage portion 128. Two outer withstand voltage portions 1271 and 1272 are arranged concentrically on the outside this intermediate withstand voltage portion 128. A plurality of narrow grooves with a rectangular planar shape are arranged on the inner side of the intermediate withstand voltage portion 128. These narrow grooves are arranged parallel to one another. The narrow grooves are filled with narrow groove withstand voltage portions 1251 to 1253 made of semiconductor layers containing P-type impurities and formed by epitaxial growth.
A thermal oxide film 114 and a PSG (Phospho-Silicate glass) film 115 are formed in that order on the surface of the epitaxial layer 112. An anode electrode 118 made of a metal thin film is disposed on the PSG film 115. An opening is formed in the thermal oxide film 114 and the PSG film 115 at the same position. The epitaxial layer 112, the narrow groove withstand voltage portions 1251 to 1253 and the intermediate withstand voltage portion 128 are exposed at the bottom of this opening and are in contact with the anode electrode 118.
The anode electrode 118 is a metal thin film forming a Schottky junction with the epitaxial layer 112 and forming an ohmic junction with the narrow groove withstand voltage portions 1251 to 1253 and the intermediate withstand voltage portion 128.
When a positive voltage is applied to the anode electrode 118 and a negative voltage is applied to the cathode electrode 119 of the diode 101 with this structure, then the Schottky junction between the anode electrode 118 and the epitaxial layer 112 is forward biased, and a current flows from the anode electrode 118 to the cathode electrode 119.
When, conversely, a negative voltage is applied to the anode electrode 118 and a positive voltage is applied to the cathode electrode 119, then the Schottky junction between the anode electrode 118 and the epitaxial layer 112 and the PN junctions between the narrow groove withstand voltage portions 1251 to 1253 and the intermediate withstand voltage portion 128 and the epitaxial layer 112 are reverse biased, and no current flows. In this situation, a depletion layer spreads from the PN junctions in the lateral direction in the epitaxial layer 112.
Conventionally, the widths and spacings between the narrow groove withstand voltage portions 1251 to 1253, the intermediate withstand voltage portion 128 and the outer withstand voltage portions 1271 and 1272 were not set with withstand voltage in mind, so that even when the epitaxial layer 112 is depleted between the long side of the narrow groove withstand voltage portions 1251 to 1253 and the inner ring circumference of the intermediate withstand voltage portion 128, the epitaxial layer 112 was not necessarily depleted between the short side of the narrow groove withstand voltage portions 1251 to 1253 and the inner ring circumference of the intermediate withstand voltage portion 128. Thus, electric fields concentrated at the locations where no depletion layer is formed, and the withstand voltage was decreased.
It is thus an object of the present invention to overcome these problems of the related art and to provide a diode element with high withstand voltage.
In order to attain the above-described object, in a first aspect of the present invention, a diode element includes a substrate of a first conductivity type, a plurality of grooves formed in a main surface of the substrate, a semiconductor filler that is made of a semiconductor of a second conductivity type, which is opposite to the first conductivity type, filled into the grooves, and an electrode film arranged on the main surface, wherein a Schottky junction is formed at a portion where the electrode film contacts the surface of the substrate, and an ohmic junction is formed at a portion where the electrode film contacts the surface of the semiconductor filler, wherein the grooves comprise a first narrow groove ring, whose planar shape is a ring and whose inner circumference is quadrilateral, and a plurality of rectangular narrow grooves, whose planar shape is a narrow rectangle, which are arranged at positions on the inner side of the inner ring circumference of the first narrow groove ring, and four sides of the rectangular narrow groove are arranged parallel to the inner ring circumference of the first narrow groove ring, wherein one intermediate withstand voltage portion and a plurality of narrow groove withstand voltage portion are constituted by the semiconductor filler filled into the first narrow groove ring and the rectangular narrow grooves, wherein the surface of the narrow groove withstand voltage portions and the substrate surface between the narrow groove withstand voltage portions is in contact with the electrode film, and wherein, the distance a between the long sides of the narrow groove withstand voltage portions opposing to the inner ring circumference of the intermediate withstand voltage portion and the inner ring circumference of the intermediate withstand voltage portion is set to substantially twice the distance b between the short sides of the narrow groove withstand voltage portions and the inner ring circumference of the intermediate withstand voltage portion.
According to a second aspect of the present invention, in a diode element according to the first aspect of the present invention, the intermediate withstand voltage portion does not contact the electrode film and is at floating potential.
According to a third aspect of the present invention, in a diode element according to the first aspect of the present invention, the grooves further include a ring-shaped second narrow groove ring enclosing the first narrow groove ring, wherein an outer withstand voltage portion is constituted by the semiconductor filler filled into the second narrow groove ring, wherein the intermediate withstand voltage portion contacts the electrode film, and wherein the outer withstand voltage portion does not contact the electrode film and is at floating potential.
According to a fourth aspect of the present invention, a diode element according the first aspect of the present invention, includes a plurality of narrow groove withstand voltage portions, wherein the narrow groove withstand voltage portions are arranged in parallel to one another at a distance d between the long sides of the narrow groove withstand voltage portions; and wherein this distance d is substantially the same as the distance a between the long sides of the narrow groove withstand voltage portions opposing to the inner ring circumference of the intermediate withstand voltage portion and the inner ring circumference of the intermediate withstand voltage portion.
According to a fifth aspect of the present invention, in a diode element according to third or fourth aspect of the present invention, a ring width w of the outer withstand voltage portion and the intermediate withstand voltage portion is substantially the same as a width y of the rectangular narrow grooves, and a distance c between the inner ring circumference of the outer withstand voltage portions and the outer ring circumference of the intermediate withstand voltage portion is substantially the same as the distance a between the long sides of the narrow groove withstand voltage portions opposing to the inner ring circumference of the intermediate withstand voltage portion and the inner ring circumference of the intermediate withstand voltage portion.
In the diode element of the present invention, the distance a between the long sides of the narrow groove withstand voltage portions opposing to the inner ring circumference of the intermediate withstand voltage portion and the inner ring circumference of the intermediate withstand voltage portion is set to substantially twice the distance b between the short sides of the narrow groove withstand voltage portions and the inner ring circumference of the intermediate withstand voltage portion.
Such a diode element is shown in FIGS. 23 and 24. FIG. 23 is a plan view of the diode element, and FIG. 24 is a cross-sectional view taken along the line Hxe2x80x94H in FIG. 23.
This diode element has a semiconductor substrate 10 of a first conductivity type, and a main surface of this semiconductor substrate is provided with narrow groove rings having a ring-shaped planar shape. A plurality of rectangular narrow grooves is arranged on the inner side of the first narrow groove ring, which is located on the innermost side. The narrow groove rings are arranged concentrically, and the rectangular narrow grooves are arranged parallel to one another. A plurality of narrow groove withstand voltage portions 251 to 253 made of a semiconductor filler of a second conductivity type are formed inside the rectangular narrow grooves, an intermediate withstand voltage portion 28 made of the semiconductor filler of the second conductivity type is formed in the first narrow groove ring, which is the innermost of the narrow groove rings, and an outer withstand voltage portion 271 made of the same semiconductor filler is formed in the narrow grooves ring to the outside thereof.
A thermal oxide film 14 and PSG film 15 are formed lamination film in that order on the surface of semiconductor substrate 10. An opening is arranged on the center of the lamination film, and at least a part of surface of the narrow groove withstand voltage portions 251 to 253 and a part of surface of the semiconductor substrate 10 where the location between the narrow groove withstand voltage portions 251 to 253 are exposed at a bottom of the opening. An electrode film 18 is formed on the surface of the lamination film where the opening is arranged. Therefore, the electrode film 18 is in contact with the narrow groove withstand voltage portions 251 to 253 and semiconductor substrate 10 at the bottom of opening. At the surface of semiconductor substrate 10 interposed between intermediate withstand voltage portion 28 and narrow groove withstand portions 251 to 253, the lamination film is arranged, so that the surface of semiconductor substrate 10 is not in contact with the electrode film 18. Furthermore, a cathode electrode 19 forming an ohmic junction with the semiconductor substrate 10 is arranged at the surface of the semiconductor substrate 10 on the side opposite to the main suface.
In this diode 1, assuming that the first conductivity type is N and the second conductivity type is P, when a positive voltage is applied to the electrode film 18 and a negative voltage is applied to the cathode electrode 19, then the Schottky junction between the electrode film 18 and the semiconductor substrate 10 is forward biased, and a current flows through the Schottky junction from the electrode film 18 to the cathode electrode 19. In this situation, also the PN junctions between the narrow groove withstand voltage portions 251 to 253 and the semiconductor substrate 10 are forward biased, but the barrier height of the PN junctions is higher than the barrier height of the Schottky junction, so that either no current at all or only a very small current flows through the PN junctions.
When a negative voltage is applied to the electrode film 18 and a positive voltage is applied to the cathode electrode 19, then the Schottky junction between the electrode film 18 and the semiconductor substrate 10 and the PN junctions between the narrow groove withstand voltage portions 251 to 253 and the semiconductor substrate 10 are reverse biased and no current flows.
Between the narrow groove withstand voltage portions 251 to 253, a depletion layer spreads from the Schottky junction between the electrode film 18 and the semiconductor substrate 10 and from the PN junctions between the narrow groove withstand voltage portions 251 to 253 and the semiconductor substrate 10 into the semiconductor substrate 10 between the narrow groove withstand voltage portions 251 to 253. On the other hand, between the inner ring circumference of the intermediate withstand voltage portion 28 and the narrow groove withstand voltage portions 251 to 253, the electrode film 18 is not in contact with the surface of the semiconductor substrate 10, and no Schottky diode is formed, so that the depletion layer spreads only from the PN junctions between the narrow groove withstand voltage portions 251 to 253 and the semiconductor substrate 10 into the semiconductor substrate 10.
Numeral 25 in FIG. 25 denotes one narrow groove withstand voltage portion of narrow rectangular shape, whose long sides are parallel and adjacent to the inner ring circumference of the intermediate withstand voltage portion 28.
Numeral 25d in FIG. 25 denotes the edge of the outward-facing depletion layer spreading in outward direction from the PN junction between the long sides of the narrow groove withstand voltage portion 25 and the semiconductor substrate 10. Numeral 28d in FIG. 25 denotes the edge of the inward-facing depletion layer spreading in inward direction from the PN junction between the inner ring circumference of the intermediate withstand voltage portion 28 and the semiconductor substrate 10. In FIG. 25, the depletion layer spreading from the Schottky junction formed between the semiconductor substrate 10 and the electrode film 18 has been omitted.
Ordinarily, the impurity concentration of the substrate is uniform, and when the same voltage is applied to the intermediate withstand voltage portion 28 and all narrow groove withstand voltage portions 251 to 253, then the width of the depletion layer spreading from the intermediate withstand voltage portion 28 is the same as the width of the depletion layers spreading from the narrow groove withstand voltage portions 251 to 253.
Consequently, if the depletion layer spreading from the intermediate withstand voltage portion 28 contacts the depletion layers spreading from the narrow groove withstand voltage portions 25 adjacent to the intermediate withstand voltage portion 28, then the depletion layers meet at a location that is in the middle between the intermediate withstand voltage portion 28 and the narrow groove withstand voltage portion 25.
Since a is the distance between the inner ring circumference of the intermediate withstand voltage portion 28 and the long side of the narrow groove withstand voltage portions 25 adjacent to the intermediate withstand voltage portion 28, the edge 28d of the depletion layer spreading in inward direction from the intermediate withstand voltage portion 28 and the edge 25d of the depletion layers spreading in outward direction from the long side of the narrow groove withstand voltage portions 25 meet at a location at half that distance a/2.
The reference numerals 701 to 704 in FIG. 25 denote corner portions, which are the vicinities of the four corners of one narrow groove withstand voltage portion 25, and these corner portions 701 to 704 are formed at the four corners of each of the narrow groove withstand voltage portions 251 to 253 respectively.
Ordinarily, the depletion layers spreading in outward direction from the long sides of the narrow groove withstand voltage portions 251 to 253 except for the corner portions 701 to 704 extend in a direction perpendicular to the long side. The depletion layer spreading in outward direction from the corner portions 701 to 704 extend not only in the direction perpendicular to the long side but also in the direction perpendicular to the short side. At the corner portions 701 to 704, an amount of depletion layer spreading in the direction perpendicular to the long side is smaller than the amount of depletion layer spreading from the long side except for the corner portions 701to 704. In the result, the outward edge 25d of the depletion layer at the corner portions 701 to 704 is rounded.
Assuming that the four corners of the depletion layer are rounded and the outward edge 25d of the outward-facing depletion layer contact the long side of the edge 28d of the inward-facing depletion layer except at the corner portions 701 to 704, at a position of the distance a/2, then virtual deficient portions of depletion layer appear in the regions close to the corner portions 701 to 704.
FIG. 26 is a partial enlarged view of the intermediate withstand voltage portion 28 denoted by numeral 30 in FIG. 25 and one narrow groove withstand voltage portion 25 opposing to its inner ring circumference. Numeral 82 in FIG. 26 denotes the edge of the outward-facing depletion layer, assuming that it has reached the distance a/2 perpendicular to the long edges in a situation in which the outward-facing edge of the depletion layer near the corner portions 701 to 704 is rounded. Numeral 81a in FIG. 26 is the line marking the position at a distance of a/2.
Numeral 81 in FIG. 26 denotes the virtual deficient portion of the depletion layer. This virtual deficient portion 81 is the region enclosed by the extending line 81b extended from the short side of the narrow groove withstand voltage portion 25, the line 81a and the edge 82 of the depletion layer.
This virtual deficient portion 81 appears at each of the four outward corners of the outward-facing depletion layer, so that there are four in total such virtual deficient portions 81.
Numeral 80 in FIG. 26 denotes the protruding portion enclosed by the line 86, which is line parting the short side of that narrow groove withstand voltage portion 25 in two equal parts, the extending line 81b and the edge 82 of the depletion layer. This protruding portion 80 appears twice at each short side of the narrow groove withstand voltage portion 25, so that there are a total of four for each narrow groove withstand voltage portion 25. The total volume of these four protruding portions 80 is equal to the total volume of the depletion layer spreading from the short side of the narrow groove withstand voltage portion 25. The total volume of the depletion layer bulging out is the same as the total volume of the above-described four virtual deficient portions 81, so that when the four protruding portions 80 are assumed to be allocated to the respective four virtual deficient portions 81, then the depletion layer spreading from one narrow groove withstand voltage portion 25 spreads to an narrow rectangular shape in a direction perpendicular to the long sides of the narrow groove withstand voltage portion 25, but does not spread at all from the short sides.
Even in this situation in which the depletion layer does not spread at all from the short sides of the narrow groove withstand voltage portion 25, the edge 28d of the depletion layer extending from the inner ring circumference of the intermediate withstand voltage portion 28 should reach the short side of the narrow groove withstand voltage portion 25, in order to deplete the substrate between the short sides of the narrow groove withstand voltage portions 25 and the intermediate withstand voltage portion 28.
When the edge 25d of the outward-facing depletion layer spreading from the narrow groove withstand voltage portion 25 and the edge 28d of the inward-facing depletion layer spreading from the inner ring circumference of the intermediate withstand voltage portion 28 meet, and the inward-facing depletion layer extends for the distance a/2 at the portion opposing to the long side of the narrow groove withstand voltage portion 25, then it extends also for the distance a/2 at the portion opposing to the short sides of the narrow groove withstand voltage portion 25. Consequently, if the distance between the inner ring circumference of the intermediate withstand voltage portion 28 and the short side of the narrow groove withstand voltage portions 25 is set to a/2, then the edge 28d of the depletion layer spreading from the inner ring circumference of the intermediate withstand voltage portion 28 reaches the short side of the narrow groove withstand voltage portion 25 in this situation, and the substrate between the short side of the narrow groove withstand voltage portion 25 and the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted.
Since the narrow groove withstand voltage portions 25 are arranged parallel each other, the depletion layer spreads also in the substrate located between the two sides of parallel adjacent narrow groove withstand voltage portions 25.
When the substrate between the narrow groove withstand voltage portions 25 and the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted, then the outward-facing depletion layer spreading from the long side of the narrow groove withstand voltage portions 25 extends for the distance a/2.
If the distance between the long sides of adjacent narrow groove withstand voltage portions 25 is set to the distance a, then the edge of the depletion layers extending from the long sides of the adjacent narrow groove withstand voltage portions 25 also spread for a/2 each, and they meet in the middle between the adjacent narrow groove withstand voltage portions 25, so that when the substrate between the narrow groove withstand voltage portions 25 and the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted, also the substrate between the adjacent narrow groove withstand voltage portions 25 is completely depleted.
As described above, the distance between the inner ring circumference of the intermediate withstand voltage portion 28 and the long sides of the narrow groove withstand voltage portions 25 is set to a, the distance between the inner ring circumference of the intermediate withstand voltage portion 28 and the short sides of the narrow groove withstand voltage portions 25 is set to a/2, and the distance between the adjacent narrow groove withstand voltage portions 25 is set to a, then the substrate inward from the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted.
If the depletion layer has spread for the distance a/2 inward from the inner ring circumference of the intermediate withstand voltage portion 28, then the depletion layer also spreads for the distance a/2 outward from the outer ring circumference of the intermediate withstand voltage portion 28.
As shown in FIG. 23, a ring-shaped outer withstand voltage portion 271 is arranged on the outer side of the outer ring circumference of the intermediate withstand voltage portion 28. If the intermediate withstand voltage portion 28 and the outer withstand voltage portion 271 are at the same potential when the depletion layer has spread for the distance a/2 from the outer ring circumference of the intermediate withstand voltage portion 28, the depletion layer spreads for the distance a/2 inward from the inner ring circumference of the outer withstand voltage portion 271.
Consequently, if the distance between the outer ring circumference of the intermediate withstand voltage portion 28 and inner ring circumference of the outer withstand voltage portion 271 is set to a, then the edges of the adjacent depletion layers meet and the substrate between the intermediate withstand voltage portion 28 and the outer withstand voltage portion is completely depleted when the depletion layer has spread outward for the distance a/2 from the outer ring circumference of the intermediate withstand voltage portion 28. When the substrate inward from the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted, then the depletion layer spreads for the distance a/2 outward from the outer ring circumference of the intermediate withstand voltage portion 28, so that in this situation, the substrate located on the inner side of the outer withstand voltage portion 271 is completely depleted.
It should be noted that if the N-type substrate on the inner side of the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted, then the narrow groove withstand voltage portions 25, which are P-type regions located on the inner side of the inner ring circumference of the intermediate withstand voltage portion 28 is completely depleted, and the depletion layer also spread into the intermediate withstand voltage portion 28.
Furthermore, if the N-type substrate located on the inner side of the inner ring circumference of any outer withstand voltage portion is completely depleted, the P-type region located on the inner side of that outer withstand voltage portion, such as the outer withstand voltage portion or the intermediate withstand voltage portion, are completely depleted, and a depletion layer spreads also to the inside of that outer withstand voltage portion.